High Level Data Link Protocol in Linux
X.25 was the first protocol suite to incorporate a generic Layer 2
protocol for its link layer definition. The generic family of Layer 2
protocols used for WANs are called High Level Data Link Protocols
(HDLCs). Originally, when layered protocols were new, “high level” meant
that the protocol was dealing with anything more abstract than the
electrical details of physical transmission. HDLC is still the basis of
many data communication protocols in common use today with the exception
of ATM and optical protocols. There are a few variants on the basic
theme of HDLC, and most of these variants are governed by ISO standards.
However, there are also a few HDLC-type protocols specified by ITU and
IEEE standards ISO 3300 and ISO 4335.See Table for a list of various
HDLC derivative protocols. It is important to point out that although
these HDLC variants are similar in the way they work, they do not expect
interoperation between variants because the framing details are
different.
HDLC Protocol Variants
HDLC Types and Configurations
This section shows the HDLC protocol in more detail. All the variants of HDLC are functionally similar. They might differ in the precise lengths of the address, control, or FCS fields, but this section is largely applicable to each variant. HDLC supports three types of stations and two types of configuration. The three types of stations are:
Primary: Controlling station on the link.
Secondary: Slaves to primary stations.
Combined: Act as either primary or secondary stations.
In addition to the three types of stations, HDLC specifies two types of configurations. The configurations can be thought of as modes of service. The two types of configurations are:
Balanced: In a balanced configuration, there are two stations connected with a point-to-point link. In this configuration, each station acts as a combined station.
Unbalanced: With the unbalanced configuration, there are two or more stations. One station acts as a primary station. Two or more other stations act as secondary stations. Figure illustrates the unbalanced configuration, and Figure shows the balanced configuration. With the balanced configuration, the stations are peers. Each station can send both commands and responses. The balanced configuration is used primarily for point-to-point links. In contrast, with the unbalanced configuration, the primary station is the controlling station on the link and the secondary stations are slaves. The primary station is the only one that can send commands, and the secondary stations answer with responses.
HDLC unbalanced configuration
Figure .HDLC balanced configuration.
HDLC Framing
As we have seen, there are many variants of HDLC. The HDLC framing detailed in this section shows framing that is generally common to all of the HDLC variants. Table shows HDLC framing that applies to most of the variants. There are three types of formats in an HDLC frame:
The primary distinguishing factor for each type of frame is the control field. The control field has different values for each type of framing, which is determined by the first 2 bits in the control field. In the 8-bit control format, bit 5 is the poll/final bit. It is treated as the poll bit if it is part of a frame sent by the primary station, and it is called the final bit if it is encountered in a frame sent by the secondary station. Although the HDLC variants are similar, the specific use of the control field is somewhat different for each type of HDLC. For example, Section on PPP shows a specific variant of HDLC called LAPB. The control field can be either 8 or 16 bits in length. Tables and show the format of the Control field for each type of frame. Table shows the information format. This format is used to transmit user data. The “I” type frames or information frames are sequenced or numbered frames for providing a connection oriented service. The transmitted frames include sequence numbers, and the response frames include expected sequence numbers.
Information Format
Tableshows the supervisory format. This format is for control functions only, primarily acknowledgment and requests for retransmissions. Outgoing information frames are not sequenced, but the n(r) field might contain the number of an acknowledged frame.
Supervisory Format
The format for commands used with the supervisory format is shown in Table
Supervisory Format Commands
Table shows the unnumbered format of HDLC. This format is used for control functions and management functions such as link establishment. Table shows some key unnumbered format commands.
Unnumbered Format
Unnumbered Commands
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HDLC Protocol Variants
HDLC Types and Configurations
This section shows the HDLC protocol in more detail. All the variants of HDLC are functionally similar. They might differ in the precise lengths of the address, control, or FCS fields, but this section is largely applicable to each variant. HDLC supports three types of stations and two types of configuration. The three types of stations are:
Primary: Controlling station on the link.
Secondary: Slaves to primary stations.
Combined: Act as either primary or secondary stations.
In addition to the three types of stations, HDLC specifies two types of configurations. The configurations can be thought of as modes of service. The two types of configurations are:
Balanced: In a balanced configuration, there are two stations connected with a point-to-point link. In this configuration, each station acts as a combined station.
Unbalanced: With the unbalanced configuration, there are two or more stations. One station acts as a primary station. Two or more other stations act as secondary stations. Figure illustrates the unbalanced configuration, and Figure shows the balanced configuration. With the balanced configuration, the stations are peers. Each station can send both commands and responses. The balanced configuration is used primarily for point-to-point links. In contrast, with the unbalanced configuration, the primary station is the controlling station on the link and the secondary stations are slaves. The primary station is the only one that can send commands, and the secondary stations answer with responses.
HDLC unbalanced configuration
Figure .HDLC balanced configuration.
HDLC Framing
As we have seen, there are many variants of HDLC. The HDLC framing detailed in this section shows framing that is generally common to all of the HDLC variants. Table shows HDLC framing that applies to most of the variants. There are three types of formats in an HDLC frame:
- Information format for carrying user data.
- Supervisory format for control functions.
- Unnumbered format forcontrol and management.
The primary distinguishing factor for each type of frame is the control field. The control field has different values for each type of framing, which is determined by the first 2 bits in the control field. In the 8-bit control format, bit 5 is the poll/final bit. It is treated as the poll bit if it is part of a frame sent by the primary station, and it is called the final bit if it is encountered in a frame sent by the secondary station. Although the HDLC variants are similar, the specific use of the control field is somewhat different for each type of HDLC. For example, Section on PPP shows a specific variant of HDLC called LAPB. The control field can be either 8 or 16 bits in length. Tables and show the format of the Control field for each type of frame. Table shows the information format. This format is used to transmit user data. The “I” type frames or information frames are sequenced or numbered frames for providing a connection oriented service. The transmitted frames include sequence numbers, and the response frames include expected sequence numbers.
Information Format
Tableshows the supervisory format. This format is for control functions only, primarily acknowledgment and requests for retransmissions. Outgoing information frames are not sequenced, but the n(r) field might contain the number of an acknowledged frame.
Supervisory Format
The format for commands used with the supervisory format is shown in Table
Supervisory Format Commands
Table shows the unnumbered format of HDLC. This format is used for control functions and management functions such as link establishment. Table shows some key unnumbered format commands.
Unnumbered Format
Unnumbered Commands
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